1. Field of the Invention
The invention generally relates to the field of test equipment for the determination of the radar cross-section (RCS) of models and, in particular, to support pylons for the model.
2. Description of Related Art
There are several ways to support radar targets on an instrumentation radar range. These include metal ogival pylons, string supports and low density foam pylons. Each approach has its advantages and disadvantages, but the major requirement of any support pylon is to have a lower radar cross-section (RCS) than the model target that is measured. Foam pylons have a low RCS because they appear to be almost transparent to the radar and, thus, are ideal for measuring small, low-weight models. With a dielectric constant of about 1.04, the surface reflection coefficient is about 0.01 or 40 dB below a metal surface. For example, the RCS of a 2-foot diameter foam pylon, 14 feet in height, can be found by reducing the RCS of a metal pylon about 40 dB. The RCS at normal incidence to a metal cylinder can be calculated by use of the formula: ##EQU1## At 400 mhz the RCS is about 20 dBsm. Subtracting 40 dB gives an RCS of -20 dBsm. The worst case occurs when the diameter of the cylinder is about one-quarter wave length, which will cause the RCS to raise by 6 dB or to -14 dBsm.
The simple cylindrical pylon's RCS may not be low enough in some test situations. However, by tapering the cylindrical pylon, the radar does not view the cylinder at normal incidence so the RCS can be significantly reduced. This concept holds true except at low frequencies where the change in diameter is less than a wavelength. But tapering has some practical limitations. The model weight determines the diameter at the top and, for most practical models, the minimum diameter is typically limited to about 2 feet. Therefore, the only alternative is to increase the diameter of the base. However, there is a limit on the available size of foam blocks. Gluing blocks together does not work well because the RCS of the glue joint is generally larger than the RCS of the pylon.
Another approach is to design the pylon to take advantage of phase cancellation by alternating surface diameters of the pylon (forming a serrated cylindrical surface) with the surfaces separated by one-quarter wave length. However, such pylons are difficult to make and the scattering from the corners is high. A better approach is to taper the pylon so that the top of the pylon phase cancels the bottom of the pylon. However, while the RCS is improved, the bandwidth is limited.
Thus, it is a primary object of the subject invention to provide a support pylon for RCS testing of a model.
It is another primary object of the subject invention to provide a support pylon for RCS testing of a model at low frequencies.
It is a further object of the subject invention to provide a support pylon for RCS testing or a model over a somewhat broad, low-frequency band.
It is a still further object of the subject invention to provide a support pylon made of foam for RCS testing of a model that is inexpensive to manufacture.